It’s normal for beginners to have this problem, so don’t look at it as something wrong as much as it is something new. It just takes some time to get used to, and learning when to tug a little and when to loosen up is an individual thing that comes with time.
What may be helpful though, is to consider that just like crochet or cross stitch, cornrowing is a similar craft. If you were braiding a rug by hand, you would want it on your lap or on your table, and have it at an angle where your hands could do the working while your arms are just there for support.
So try sitting down and turning the practice head at an angle that you are braiding towards yourself. Your arms can start out somewhat extended, but as you’re getting further along the row, your arms should be moving closer to your body. This is ergonomically better than trying to do the entire braid with the arms held up away from the body.
Also, there is some relation between the width of the row, the thickness of the hair, and how much hair you should pick up when you’re adding to the middle strand. Remember that it’s not an exact science. Sometimes you’ll need to leave some slack, and then start picking up after one or two regular stitches. Go with the shape of the head. Around curves is where people usually need to make some slack.